1. Field of the Invention
The invention relates to an energy-conserving device of an illumination system, in particularly to an energy-conserving device employed for high intensity discharge (HID) lamps in hopes of saving energy by means of adjusting illumination intensity.
2. Description of Prior Art
HID lamps have been widely used as lighting sources due to its high efficiency and long service life, which are especially utilized in outdoor areas, such as roads, sports arena, supermarkets, etc. As show in FIG. 1, a conventional HID lamp includes a discharge light bulb 640, a trigger 630 for initiation of electrical discharge, a ballast 610 for limiting current that passes through a bulb 60, as well as a compensating capacitor 620 for power factor correction. Generally, the illumination intensity of the current HID lamps keeps the same, regardless of the required brightness of the illumination, therefore resulting in wasting large amount of energy.
To address the shortcoming, Chinese patent No. 200420081845.x discloses a power-variable inductance ballast device. As illustrated in FIG. 2 of the patent, an additional inductance ballast 720 is connected in parallel to an inductance ballast 710 via a tap. The power level is controlled through a switch 730 connecting each of ballasts 710 and 720 so as to alter the output power of an electrical discharging light bulb 740. However, this solution increases the complexity of wiring of the inner circuitry. Furthermore, when the system works at different power levels, its circuitry may not match its power factor due to the limitation of the compensation capacitor. So, it is hard to reach a desirable effect of energy conservation. In addition, when the power level is switched, the system will experience a switching period. In essence, it's a power down process, which might result in collapse of arc and interruption of illuminating. To avoid this incident, it is necessary to shrink the switching period. A high-quality controller must be used for this purpose, which results in lower cost effectiveness and raises the failure rate of the product.